The central hypothesis of our Program Project is that the mechanisms primarily responsible for normal and malformed heart development are not wholly intrinsic to cardiogenic mesoderm. It is proposed that instructive cues carried by successive waves of invading mesenchyme. It is proposed that instructive cues carried by successive waves of invading mesenchyme are key to the remarkable transformation of the vertebrate heart from a simple, peristaltic vessel to a complex, multi-chambered pump. The sources from which these mesenchymal cells migrate are diverse and include the coelomic mesothelium (source of pro-epicardium) and the neural tube (neural crest). The Program seeks to evaluate the impact of these migrations in orchestrating multiple facets of cardiovascular pattern formation including atrioventricular (AV) conduction system development, epicardiogenesis, outflow tract formation and deployment of the coronary arterial bed. The theme of Project I is the genetics of normal and abnormal electrical linkage in the human AV conduction system. A key hypothesis here is that Nkappax2.5 mutation results in abnormal patterns of cellular recruitment to the conduction system. Project II will investigate the role of invasive, epicardial-derived mesenchyme (EPDCs) and WNT-signaling in orchestrating pattern formation in the AV conduction system. A principal hypothesis in this study is that interaction between EPDCs and embryonic myocardium is intimately tied to the induction of gene programs associated with recruitment of specialized conductive myocardium is intimately tied to the to the induction of gene programs associated with recruitment of specialized conductive myocardium (e.g. Connexin 40/42 and Nkx2.5). In Project III, the role of gap junctional communication between neural crest and epicardial-derived cells is studied. The goal here is to determine how intracellular dialogue coordinates the motility of these cohorts and ultimately influences outflow induction of epithelial- mesenchymal transformation at the epicardium. The overall goal of this project is to initiate the intra-cardiac invasive potential of these mesenchymal cells. This Program integrates diverse and state-of-the-art methodologies (including transgenic gene targeting, gene microchip array screening, ultrarapid dynamic imaging and genetic linkage analysis of human kindreds) and established investigator expertise to create a unique research environment to determine the underlying mechanisms that determine structural malformation of the human cardiovascular system.